The invention deals with a band-type filter for recovering or removing foreign material particles from liquid baths. A bath tub contains the liquid and guide rollers are arranged in the tub for a filter band introduced from above into the tub bath. The filter band is guided so as to be spaced above the tub bottom, and leaves the tub again from the top. The filter band is configured as a driven endless screening tissue band or as a filter fleece to be unreeled from a roller arranged externally of the tub and resting on an endless driven support belt. A negative pressure chamber receiving the filtered liquid is located below the filter band.
In known band filter installations of this type, the filter band placed upon the perforated bottom plate is respectively pulled forward in steps across the bottom plate for introduction of fresh band segments, after the pressure acting upon the band upper surface produced by the negative pressure in the negative pressure chamber has been removed. With this, the filtering operation is interrupted for as long as needed until fresh band segments have been introduced. Because of this circumstance, on the one hand unpurified liquid flows past the filter, and, on the other hand, the necessity of continuous switching off and on of the filtering process requires considerable resources in terms of apparatus. This start-stop operation entails an additional disadvantage in that directly after the filtering process has been switched off, the filter band lying on the bottom floats or rises towards the top due to the flow reversal in the region of the filter bottom, and also because the filter band tends to float because of its low specific density. This effect is assisted by rising air bubbles. In order to counteract this disadvantage, one has equipped such filters with rods placed transversely to the travel direction of the filter band, which load the filter bands by their weight and thus are meant to prevent the upward floating. During the course of the stepwise pulling forward of the filter band under this load, it is frequently damaged through formation of holes or tears. The rods, because of their weight, increase the friction between the filter band and the base plate, for which reason only special high tensile strength filter bands can be used, which require an increased expenditure.
Therefore, it was already proposed to support the filter band across the base plate on stationary rollers in order to enable a continuous operation. The resting of the filter band upon such rollers however also entails difficulties because the band tends to sag between the rollers and thus no filtering circuit with constant and thus controllable spacing from the pass through-apertures for the filtered fluid in the base plate can be created. Also, the design of the rollers themselves and their arrangement is expensive and requires additional maintenance. It must be added that the side edges of the filter band cannot in actual practice be adequately sealed against the direct passage of unfiltered liquid into the negative pressure chamber and therefore the filtering efficiency of such installations is impaired.